I have just started putting the parts back together after a carbon and injector clean and was wondering at what point would you guys do the cylinder purge?
I have the injectors and fuel rails back on at the moment. Advice?
You can do it anytime after you’re fully done with the carbon clean and will not be doing any further work that can make it’s way into the cylinders.
Will petrol flow out of the valve where the fuel line splits, just that when I reconnected the battery after a charge the pump at the tank primed and spat fuel into the valley?
Pull the fuel pump fuse 
You can just unhook the fuel pump. Just unplug the wire harness. If you want to go an extra step after that you can unhook the fuel line from the rail.